Foundation/Slab Design Un-Heated Building

[jchi]

How do others address designing foundations/slabs in unheated PEMB. Here is my problem - 4' frost, PEMB, unheated, large horizontal thrust reaction.

My concerns:
1. Slab moves due to frost heave - do you pour foundation wall with curb and attach panel to curb, let slab float - do you try to tie slab to foundation wall with rebar -
2. Hairpin bars from columns restrict slab movement - pour tie-beams that extend to frost in lieu of hairpins
3. Try to remove frost action by remove one of three componetnts of frost action, large amount of structural fill or graded gravel.
4. Will the subgrade freeze under the slab with-in a un-heated building - depends on moisture content.

What do others do.

At the highest cost - pour curb, float slab and provide tie-beams extending to frost depth. I feel this is the best option...

Jeff

 

[SteelPE]

Have you checked out ASCE 32? This will basically cover your item #3. I have never used the standard but I have seen at least one small structure that has.

 

[hokie66]

My preferred footing type for a gable frame consists of a pair of bored piles aligned parallel with the frame, with the column supported on the pile cap. The bored piles and pile cap form a rigid frame and distribute the thrust reaction to the soil. Second preferred is a tension tie across the building. Least preferred, and only used for very minor structures where the slab is never likely to require modifications, is hairpins to the slab.

 

[JAE]

We've used the #3 option along with a thicker mat foundation - this was for a shower building in a park/camp area which was not heated in the winter. Also had lots of masonry walls on the interior dividing up the shower areas. So a floating slab would have wreaked havoc on the masonry.

 

[dik]

Only 4'... that's not so bad... with sunlight on the roof, it will help moderate things in the winter and if you design for 4' it should be adequate.

Without insulation in the roof, it's going to awfully warm in the summer. Automatic louvres may be adequate, to keep the heat in, in the winter.

You can remove the frost heave susceptable soil; drainage of any moisture will be an issue, in particular if not insulated.

Perimeter foundations should be taken down to below frost level.

You can provide rigid insulation under the slab to permit geothermal heat to rise and keep the frost from going down. This is OK if not a cold storage building with continuous frost conditions. This may mitigate the excavation and compaction.

You can similarly insulate the foundations and run ties in the slab for lateral thrusts from the PEMB. You may be able to do a stiffened slab foundation.

Dik

 

[jchi]

I should have the soils report in the next week. They want to pour concrete 1st of October and still have not provided the report... How do they expect me to complete a design.

Now - They want to pour this 12,000sqft building foundation and slab in northern Montana this October and November and wait to erect the building in spring. Therefore the the slab will be exposed the first year.

Do I use air in the entire slab? - tough to trowel finish.
Pour a curb and float the slab.
Bury tie-beams to frost depth.

Insulation under entire slab is an option, but high $$$ and still may see frost.

Jeff

 

[dik]

...but, you may save 4' of excavation and backfill...

Dik

 

[jchi]

Good point Dik - I will look at that.

What do most use 1" rigid insulation = 1' depth, thus 4'=4" of rigid insulation.

Jeff

 

[dik]

You have to extend it 4' around the building, too... If lighty loaded, you may get by with a 5" slab and 12x18 edge stiffener, or something of that ilk... larger edge stiffener at column locations and run the horizontal thrust bars/hairpins into the slab. I've used that approach for northern storage buildings.

Dik

 

[SteelPE]

I think you have to extend the insulation even further (than 4') at the corners of the building. ASCE 32 should explain everything including how much R value you will need.